This invention relates to a method for conditioning coal or other carbonaceous material prior to physical separation. In particular, the pretreatment involves the use of supercritical fluids in conditioning the carbonaceous material.
Several factors limit the increased efficient utilization of coal. The gaseous release of the sulfur and nitrogen species in coal upon combustion has been and remains one of the most important limitations to increased utilization. Also the presence of high levels of mineral matter prohibit the use of coal, especially in markets traditionally dominated by petroleum products and natural gas.
Commercial approaches to coal cleaning rely on differences in the physical properties of the coal and mineral matter to reject the undesirable portion of the coal. Many coals are known to be unresponsive to such techniques due to a variety of reasons, one of which is the incomplete segregation of mineral matter from the organic matrix of the coal.
Chemical treatment of coal after physical beneficiation has been used in coal cleaning. Size reduction and classification in the physical process steps improve the activity of any chemical treatment that is later used. Recently, however, there have been suggestions to reverse this order of treatment to improve grindability, increase coal/ash fusion temperatures and preserve combustable volatiles. In one instance, a carbon dioxide and water mixture was used for the pretreatment of coal prior to its physical benefication. The coal was treated substantially below supercritical density and there was no significant extraction of soluble components into the carbon dioxide and water.
It is known that certain gas phases maintained near to supercritical conditions are capable of taking up large amounts of solutes from liquid or solid materials. When conditions such as temperature or pressure are reduced to below critical, a substantial decrease in solubility results. Also increases, particularly in temperature, to well above critical likewise reduce solubility in the supercritical gas. For purposes of this application, the terms "supercritical solvent", "supercritical phase", or "supercritical fluid" refer to a gas or gas mixture, in some instances with solute, at or above critical temperature and critical pressure.
There is increasing interest in the identification, development, and characterization of new coal derived fuels. Not only is there interest in beneficiating coal by the removal of ash and sulfur but also there is interest in beneficiating various char products from pyrolysis and gasification processes. Since a large portion of the calorific value of the coal ends up in the char, the benefication of this material can produce an important fuel. Removing the ash from char can produce serious difficulties in that char formation may fuse the organic and mineral matter and retard separation.
Therefore, in view of the above, it is an object of the present invention to provide an improved method of beneficiating carbonaceous material.
It is a further object to provide a method of reducing the ash content of a coal or char.
It is a further object of the invention to provide a method of removing ash and sulfur from carbonaceous material without comminution to fine particles.
It is also an object to provide a premium ash-free fraction in a process for the reduction of ash in a carbonaceous material.